The invention relates to a machine for making flat bottom bags, in particular paper bags, of the type including a drum onto which a blank bag is driven and follows various folding and gluing operations in order to form the bag.
Flat bottom bags have been known for many years now, used for example to carry provisions or shopping in stores. Such a bag comprises a tubular envelope closed at one end by a flat bottom. The flat bottom is obtained by the pressing, the folding down of the flaps from the envelope and the gluing of these flaps together.
The manufacture of these bags has been more or less automated and according to various techniques. One of them is detailed, for example, in document U.S. Pat. No. 3,554,099. The machine described in this document shows that a blank is first of all formed and presents two skins connected together along two opposite edges by two bellows. The blank is placed flat on a table, one of the skins, called the lower skin, being against the table, the other skin being above and the bellows being pressed down between the two skins. The blank is driven in translation on the table in a direction parallel to the bellows towards a peripheral drum from which various stations are distributed.
First scoring means make a first fold line at the bellows, and a second fold line at a predetermined distance from the first line, in a direction perpendicular to the bellows. The scoring means comprise a rotary blade holder and additional slots carried by a counter-rotating drum. As it progresses, the front of the blank is gripped between the drum and a roller. The drum comprises front pinching means to drive the blank by pinching the front edge of the lower skin. To facilitate the separation of the two skins, the opening roller includes suction means which maintain the upper skin against the roller after it has passed between the roller and the drum. Set back in relation to the front pinching means, first lateral pinching means pinch the lateral edge of the blank at the junction between the bellows and the lower skin. Symmetrically, two pinching means pinch the lateral edge of the blank at the junction between the bellows and the upper skin. When the blank continues to pass between the roller and the drum, the lateral pinching means move away from each other, opening the blank and forming a fold on the bellows in a radial plane of the drum and the roller. The two pinching means release the blank whilst in the continuation of the movement of the drum which drives the blank, spatulas press down the open part towards the centre. The upper skin is folded towards the rear at the second fold line.
In the continuation of the operations, second scoring means mark two transverse lines, on either side of the second fold line. One of these lines coincides with the first fold line. Then, adhesive is applied to the two flaps delimited by these fold lines and they are pressed down one onto the other to close the flat bottom and thus form the bag.
Such a machine allows a high production rate to be attained as all the manufacturing operations are automated. However, it is designed for a single bag format. When another format is to be produced, many parts must be changed and laborious adjustments must be made. Also, certain characteristics of the bag produced depend on the production rate in such a way that it is difficult to anticipate these characteristics by an adjustment when the machine is stopped. The characteristics must be checked when the machine is in operation, then new adjustments must be done on the stopped machine before a new validation in operation.
The aim of the invention is therefore to provide a machine for making flat bottom bags allowing the bag format to be changed very rapidly and easy adjustment of the characteristics of the bags produced.